The application of an adaptive array antenna to mobile communication systems has been studied. An adaptive array antenna has already been put to practical use in base station apparatuses for PHS (Personal Handyphone System). In an adaptive array antenna, outputs from a plurality of antenna elements of the array are multiplied by respective weights (a combination ratio) and are then combined, so that interference waves are removed and a desired output is obtained. At this time, it is important to provide a set of appropriate weights according to a receiving environment in which the adaptive array antenna is placed. However, it is known that when the adaptive array antenna is placed in a communication environment with large variations in the reception level, in which fading or the like occurs, or when the directions from which a desired wave and an interference wave come are, nearly the same, the weight calculation becomes unstable or results in an erroneous convergence of the set of weights.
Japanese patent application publications No. 9-260941, No. 10-51221, No. 6-120856, No. 11-234035, etc. disclose conventional technology associated with the above-mentioned system.
Particularly, “Pilot Symbol-Assisted Decision-Directed Coherent Adaptive Array Diversity for DS-CDMA Mobile Radio Reverse Link”, IEICE TRANS, Vol. E80-A, No. 12, December 1997 discloses an example of W-CDMA (Wideband Code Division Multiple Access) combining reception apparatuses which use an adaptive array antenna. The structure of the combining reception apparatus is shown in FIG. 5.
The combining reception apparatus 1 of FIG. 5 is provided with a common unit 2 and a digital signal processing unit (DSP) 5. The common unit 2 is provided with a plurality of antennas al to an each of which receives an incoming received wave, a frequency changing unit 3 for transforming high-frequency signals applied thereto into baseband signals, and an analog-to-digital (AD) conversion unit 4 for converting the analog signals into digital signals. The digital signal processing unit 5 is provided with a plurality of finger units f1 to fn each of which performs demodulation processing for each multipath and a multiplexer 6 for multiplexing signals output from the plurality of finger units f1 to fn into a single signal.
A received wave, which has come to the combining reception apparatus 1, is received by the antenna a1, is converted into a baseband signal by the frequency changing unit 3, is converted into a digital signal by the A/D unit 4, and is then input to the digital signal processing unit 5. The input signal is demodulated on a path-by-path basis by the plurality of finger units f1 to fn. Signals from the plurality of finger units f1 to fn are then multiplexed into a single signal by the multiplexer 6 and the signal is output to an external channel decoding processing unit not shown.
FIG. 6 is a block diagram showing a detailed structure of the finger unit f1 of FIG. 5. The finger unit f1 is provided with a reverse spreading unit 7 for performing reverse spreading processing on the received signal from each of the plurality of antennas, a weight calculation unit 8 for calculating a set of weights from reverse-spread signals output from the reverse spreading unit 7, a plurality of multipliers each for multiplying a corresponding reverse-spread signal by a corresponding calculated weight, and a combining unit 9 for combining resultant signals from the plurality of multipliers into a single signal. The weight calculation unit 8 performs the weight calculation according to an interference removal algorithm, such as LMS (Least Mean Square), which is excellent at removing interference. The other finger units have the same structures as the finger unit f1.
The weight calculation unit 8 can calculate the set of weights having a directional characteristic which directs a null point in the direction of an interference wave by using the interference removal algorithm. Therefore, the plurality of finger units f1 to fn can output respective composite signals in each of which the interference wave is removed.
However, a problem with the prior art combining reception apparatus is that although it has a sufficient ability to remove interference waves by using the LMS algorithm, in a communication environment where the reception level varies greatly or when the directions from which a desired wave and an interference wave come are nearly the same, the convergence of the weight calculation may be extremely slowed down, the weight calculation may become unstable, and an erroneous convergence of the weight calculation may occur. When the weight calculation unit 8 uses an FFT (fast Fourier transform) algorithm or a maximal-ratio combining algorithm that provides a rapid convergence, the prior art combining reception apparatus cannot sufficiently remove interference waves, as a natural result.